1. Field of the Invention
The present invention relates to encoders, and more particularly to a method for realizing high accuracy encoders at a low cost by interfacing low-cost encoders with a microprocessor. The microprocessor is programmed to compile a table of set points which correspond to the actual spacings of the encoder. If the shaft which the encoder is monitoring maintains a constant speed, a certain number of encoder lines will be detected at a designated time interval or count. A variation from the designated count indicates a variation in the speed of the shaft. Upon detecting the variation in speed, a signal is sent to a motor to selectively influence the speed of the shaft, so that a constant shaft speed is maintained.
2. Discussion of the Related Art
A major expense in the production of closed loop velocity or position servos is the cost of an encoder. Very accurate encoders (Heidenhain) are priced at a few thousand dollars; encoders of medium accuracy cost a few hundred dollars and low accuracy encoders cost as little as 25 dollars. To approximate costs, an increase of one order of magnitude in encoder accuracy increases the price by one order of magnitude.
Thus, a need exists for an autocalibration technique that will provide on-line calibration of low accuracy encoders so that, with only modest increase in cost, they provide the accuracy of the high-priced encoders. By autocalibration, it is meant calibration without comparison to a high accuracy encoder.
The prior art discloses encoders and methods for improving the accuracy. For example, U.S. Pat. No. 4,593,193 to Michaelis discloses an apparatus and method wherein a servo controller uses generated pulses to calibrate an encoder. A counter is used to keep track of encoder pulses, and a memory stores a calculated error of the pulse. U.S. Pat. No. 4,224,515 to Terrell discloses a high-accuracy optical shaft encoder system wherein an encoder outputs a sine wave. The sine wave is compared to a reference sine wave from a frequency generator and is fed back to control the encoder. U.S. Pat. No. 4,633,224 to Gipp et al. discloses an absolute and incremental optical encoder wherein an algorithm is taught which improves encoder accuracy by using an encoder's absolute position signal and incremental position signal and then comparing these to a stored value. U.S. Pat. No. 3,998,088 to Kazangey discloses a testing apparatus for an incremental shaft encoder wherein a gyroscope is used to accurately test the encoder. U.S. Pat. Nos. 4,792,672 to Spies and 4,806,752 to Fischer each disclose an incremental encoder with a clamping device and a laser rotary encoder.
The concept of using a look-up table for calibration purposes is disclosed in U.S. Pat. No. 4,873,655 to Kondraske wherein a sensor conditioning method and apparatus calibrates a sensor by a look-up table generated by a microprocessor.
The prior art, however, fails to provide an autocalibration technique for increasing the accuracy of a low cost encoder so that it can function with the accuracy of a high cost encoder.